1. Field of the Invention
This invention concerns the purification of hexamethylenediamine (HMD) using ammonia as an internal refrigerant to crystallize HMD from its melt. The crystals are then washed with refined molten HMD following which the crystals are melted and refined molten HMD is recovered.
2. Description of the Prior Art
The source of the hexamethylenediamine (HMD) which can be purified by the process of the present invention includes HMD from the catalytic hydrogenation of adiponitrile in the presence of ammonia using known catalysts, e.g., nickel, cobalt, or mixtures thereof usually precipitated on a finely divided carrier and Raney nickel catalysts. Exceptionally effective catalysts include iron compounds as disclosed in U.S. Pat. No. 3,696,153 issued Oct. 3, 1972 and assigned to E. I. du Pont de Nemours and Company. The process of the present invention is particularly suited for the purification of product produced by the iron catalyzed hydrogenation.
U.S. Pat. No. 3,370,082 issued on Feb. 20, 1968 discloses a method for the purification of adiponitrile which involves cooling the adiponitrile optionally with an internal coolant which is immiscible therewith, e.g., ethane, ethylene, propane and the like. The patentees suggest that the mother liquor adhering to the crystals which are separated after evaporation of the internal refrigerant can be displaced by washing with a suitable liquid including pure adiponitrile obtained from the melting of crystalline adiponitrile. Certain methods for recycle in a process for the purification of adiponitrile are disclosed in U.S. Pat. No. 3,839,408 issued on Oct. 1, 1974 and involve the continuous purification of adiponitrile by indirectly cooling crude adiponitrile in a crystallization zone, recovering the crystals from the crystallization zone, washing the crystals thus obtained with liquid adiponitrile, and returning the separated adiponitrile mother liquor to the crystallization zone. Although the patentees disclose the use of ammonia as a coolant, the use thereof is indirect, i.e., in heat exchange relationship with the adiponitrile and is not placed in direct contact with the nitrile.
The above discussed art does not suggest the use of ammonia as an internal refrigerant for the purification of HMD. Usually HMD has been purified by distillation. However, distillation is expensive and difficult because the impurities in the HMD and the HMD have very low relative volatilities. In addition, the elevated temperatures associated with distillation tend to accelerate decomposition of HMD.